The present invention relates generally to force sensing transducers and, in particular, to a nonthreaded load sensing probe for measuring the work forces present in machines.
The ability to measure force or load is a vital part of many industrial processes. Recognizing that a structure undergoes a dimensional change when subjected to a load, a measure of the dimensional change in a structure can accurately indicate the amount of load or force applied thereto. Such measurement is usually made through the use of a wire, foil, or semiconductor element intimately bonded to the structure. By measuring the change in the electrical resistance of the element, which resistance is a function of the change in cross-section of the element due to alterations of a structure, it is possible to accurately measure the load applied to the structure.
Many transducer devices have been developed for attachment to structures for measuring the amount of stress applied thereto. The U.S. Pat. No. 3,444,390 to Breidenbach et al. and the U.S. Pat. No. 4,171,646 to Dybel et al. both disclose measuring devices for use on machine tools such as forges, presses and the like. Generally, such prior art devices are not sufficiently sensitive or physically constructed to be readily adaptable for use in areas of a machine near the tool in order to directly measure forces on the tool. For example, the transducer arrangements disclosed in the above-mentioned patents are incapable of directly measuring the working load on a tool. This inability to directly measure tool loading is a particularly acute problem when it is desired to monitor the individual loads on a plurality of tools carried on a common base and caused to do work simultaneously.
Ideally, the apparatus for measuring the work forces in a machine tool may be mounted substantially or totally in a machine so that it is protected from physical damage resulting from inadvertent impact or from contamination by debris or lubricants. It is also desirable that the load measuring apparatus be accessible for removal, inspection, maintenance or repair.
The U.S. Pat. No. 4,280,363 issued to Johansson shows a measuring shaft adapted to be fixed in a hole formed in an object to be measured. The shaft is equipped with strain gauges or similar devices for sensing a load and the inner end of the shaft is threaded to engage with threads provided in the bottom of the hole. The outer end of the shaft is fixed in relation to the hole so that the shaft, like a part unseparated from the walls of the hole but nevertheless detachable, accompanies the motions of the hole.
The U.S. Pat. No. 4,412,456 issued to Wilhelm et al. shows a load sensing probe which has a split plug including a slot defining two sensing arms, which arms are pushed outwardly into the engagement with an accommodating cavity formed in a back-up member of a machine tool. The probe is sensitive to relatively closing movement of the cavity caused by the compressive strain in the back-up member.
The U.S. Pat. No. 4,429,579 issued to Wilhelm shows a tie rod load sensor for a die-casting or other machine which has an axially extending bore formed in the tie rod to receive a strain gauge sensing member. One end of the sensing member is countersunk to receive a ball which concentrates the load when it is forced against the bottom of the bore by a bolt threaded into the opening of the bore. The sensing member includes a narrow central shaft portion upon which a plurality of strain gauges are mounted. The bolt is utilized to preload the sensing member such that the strain gauges mounted thereon will measure changes in the tension in the tie rod.
The U.S. Pat. No. 4,526,044 issued to Moser, et at. shows a load sensing probe for disposition in a cavity in a back-up member. A centrally disposed bore extends through a generally cylindrical body member having first and second end regions of substantially equal outside diameter and a third intermediate region of reduced diameter. A first pair of slots extend axially inwardly into the intermediate region and a second pair of slots extend from the second region into the intermediate region and are disposed at a right angle to the first pair of slots. A threaded rod having a frusto-conical surface extends through the centrally disposed bore and includes a threaded fastener. This assembly may be utilized to apply outwardly directed forces to the body of the probe and retain it within the cavity of the tool. Strain gauges are secured on a radially extending surface of one of the end regions and measure the dimensional change of the cavity. Opposed flats on the opposite end of the body member permit rotation of the body member in the cavity during installation.